Applications, toolkits, and printing commands select the protocol to use for client-to-server communications, based on the value of the printer-uri-supported attribute that is found in the printers.conf database entry for the specified print queue. This value is automatically generated and populated when access to a remote print queue is configured by using the lpadmin command or when an application uses the interface that is being referenced to perform an action or actions.
For example, when printing a document while using Mozilla, the path that is followed to gain access to the printer is:
Mozilla -> lp -> PAPI
For more information, see the printers.conf(4)
The lpadmin command generates printer-uri-supported values using the following format:
To determine which URI to use in the printers.conf database, the lpadmin command probes the remote print service. If the remote print service supports IPP, this protocol is preferred over the BSD protocol. However, you can override automatic protocol selection by specifying the -soption with the lpadmin command, then supplying a different URI. For more information, see the Printer URI Formats That Are Supported.
The LP print subsystem uses the BSD print protocol, also referred to as the RFC-1179 protocol, and raw Transmission Control Protocol (TCP), the basic communication language or protocol of the Internet, to communicate with the printer. If you are setting up a printer that includes printer vendor documentation, information about which protocol to use is provided. The TCP protocol is the protocol that is most often used for printing on the network.